The pygidium is the posterior body part or shield of crustaceans and some arthropods, such as insects and the extinct trilobites. It contains the anus and, in females, the ovipositor. It is composed of fused body segments, sometimes with a tail, and separated from thoracic segments by an articulation.

In arachnids, the pygidium is formed by reduction of the last three opisthosomal segments to rings where there is no distinction between tergites and sternites. A pygidium is present in Palpigradi, Amblypygi, Thelyphonida, Schizomida, Ricinulei and in the extinct order Trigonotarbida. It is also present in early fossil representatives of horseshoe crabs.

"Pygidium" is also a superseded genus of fish of the family Trichomycteridae. Most species of this genus have been reassigned to the genus "Trichomycterus".


In trilobites, the pygidium can range from extremely small (much smaller than the head, or cephalon) to larger than the cephalon. They can be smooth, as in order Asaphida, or spiny, as in order Lichida.


Shultz, J.W. (1990). Evolutionary Morphology And Phylogeny of Arachnida. Cladistics 6: 1-38.

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  • Pygidium — Py*gid i*um, n.; pl. {Pygidia}. [NL., fr. Gr. ?, dim. of ? the rump.] (Zo[ o]l.) The caudal plate of trilobites, crustacean, and certain insects. See Illust. of {Limulus} and {Trilobite}. [1913 Webster] …   The Collaborative International Dictionary of English

  • Pygidĭum — Pygidĭum, Afterdecke, hinterer Theil des Leibes, bes. bei zweiflügeligen Insecten …   Pierer's Universal-Lexikon

  • Pygidium — Pygidium, Anallappen am Hinterende der Annelida (Ringelwürmer) …   Deutsch wörterbuch der biologie

  • pygidium — [pī jid′ē əm] n. pl. pygidia [pī jid′ēə] [ModL < Gr pygidion, dim. of pygē, rump < IE base * pu , to swell up > PUPA] Zool. the end division of the body of a trilobite or of certain annelids, crustaceans, or insects …   English World dictionary

  • Pygidium — Als Pygidium (von altgriechisch πῡγίδιον Steiß) werden in der zoologischen Anatomie bezeichnet: Der letzte, nicht segmentartige Körperabschnitt der Ringelwürmer (Anneliden), siehe Pygidium (Anneliden) Das letzte Tagma (Körperabschnitt) mancher… …   Deutsch Wikipedia

  • Pygidium — Le terme de pygidium ou telson désigne l ensemble des tergites abdominaux caractérisés par l absence d appendices natatoires ou ambulatoires. Anatomie d un trilobite …   Wikipédia en Français

  • pygidium — noun (plural pygidia) Etymology: New Latin, from Greek pygidion, diminutive of pygē rump Date: circa 1849 a caudal structure or the terminal body region of various invertebrates • pygidial adjective …   New Collegiate Dictionary

  • pygidium — n.; pl. dia [Gr. pygidion, narrow rump] 1. (ANNELIDA) The telotroch and anal region behind it, bearing cirri. 2. (ARTHROPODA: Insecta) The last dorsal segment of the body; pygal, pygidial adj …   Dictionary of invertebrate zoology

  • pygidium — pygidial, adj. /puy jid ee euhm/, n., pl. pygidia / jid ee euh/. Zool. any of various structures or regions at the caudal end of the body in certain invertebrates. [1840 50; < NL < Gk pyg(é) rump + idion dim. suffix] * * * …   Universalium

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